Monoclonal antibodies for the different chains of chick type VI collagen

Alternative splicing of VWFA modules generates variants of type VI collagen alpha 3 chain with a distinctive expression pattern in embryonic chicken tissues and potentially different adhesive function

Type VI collagen, a ubiquitous extracellular cell adhesion molecule, is formed by heterotrimeric monomers which associate into dimers and tetramers and assemble into larger oligomers constituting the 100 nm-long periodic microfilaments of connective tissues. One distinctive structural characteristic of type VI collagen is represented by an alpha 3 chain with a much larger molecular mass compared to the other two chains and with an extensive size heterogeneity, exemplified by the separation into up to five polypeptides in SDS-PAGE. There is evidence that the alpha 3(VI) mRNA can undergo alternative splicing of three VWFA modules at the 5'-end, potentially resulting in the expression of protein variants. Here we report that alternative splicing of alpha 3(VI) mRNA in chicken embryo did not result in the absolute predominance of a particular alpha 3(VI) form in any tissue; instead, the expression of variants including exons A9, A8 and A6 increased with age. In addition, these variants had a more restricted tissue distribution pattern compared to variants including only constitutive exons: A9+ were the rarest and were present almost exclusively in skin and skeletal muscle; A6+ were expressed in several of the examined tissues with local variations; A8+ had intermediate levels and were less widely distributed than A6+ variants. Quantitative densitometric scanning of immunoblots of type VI collagen purified from gizzard and stained with VWFA module-specific antibodies indicated that the polymorphic migration pattern of alpha 3(VI) polypeptides is contributed by concurrent or independent splicing of two exons (A8 and A6) and probably by processing and/or proteolysis at the N- and C-terminus. Three exon-specific recombinant polypeptides were examined in cell adhesion assays, and A6 appeared to be the most active, particularly at low substrate concentrations. The adhesion to the recombinant modules was not abrogated by EDTA nor by mAbs against the integrin beta 1 or alpha 2 subunits. Over all, these results suggest that the splicing of the alpha 3(VI) mRNA and the tissue distribution pattern of type VI collagen variants, apart from promoting cell adhesion to different extents, might also affect additional structural as well as functional properties of this molecule, including microfilament formation and interaction with other extracellular matrix molecules.

Secretion and matrix assembly of recombinant type VI collagen

A monomer of type VI collagen is composed of three different chains of 140 (alpha 1), 130 (alpha 2), and 250-350 kDa (alpha 3). Monomers assemble into dimers (6 chains) and tetramers (12 chains) that are stabilized by disulfide bonds and, once associated one to another, give rise to a microfilamentous network in close apposition with cell surfaces and banded collagen fibers. We have derived murine NIH/3T3 cell lines that were transfected with the cDNAs for the three chains and that constitutively expressed chicken type VI collagen. Cotransfection was efficient because, in three out of six isolated cell lines, all chicken chains were expressed. Southern blotting demonstrated that several copies of each cDNA were integrated approximately in equal number. Expression of the three polypeptide chains was consistent with the levels of the respective mRNAs. The three chicken chains assembled by disulfide bonding to form correctly folded triple helical aggregated composites with sizes corresponding to type VI collagen monomers, dimers, and tetramers. These functional recombinant assemblies were secreted and became incorporated into the extracellular matrix, where they formed an extensive fibrillar network.

Collagen type VI in neural crest development: distribution in situ and interaction with cells in vitro

We have examined the spatio-temporal distribution of collagen type VI (Col VI) during neural crest development in vivo and its ability to promote neural crest cell attachment and migration in vitro. An affinity purified antiserum and chain-specific monoclonal antibodies against chicken Col VI were employed to immunolocalize the collagen in tissue sections and by immunoblotting. At stages of initial neural crest cell migration, the alpha 1(VI) and alpha 2(VI) chains were immunolocalized in apposition with basement membranes of the neural tube, somites, notochord and ectoderm, whereas no immunoreactivity was seen for the alpha 3(VI) chain. Immunoblotting analysis confirmed the expression of alpha 1(VI) and alpha 2(VI) chains and the lack of detectable immunoreactivity for the alpha 3(VI) chain at these early phases of neural crest development. Conversely, at advanced phases of migration and following gangliogenesis, expression of alpha 3(VI) chain coincided with that of alpha 1(VI) and alpha 2(VI) chains in apposition with basement membranes, around the dorsal root ganglia, and in fibrillar arrangements within the developing dermis and ventral sclerotome. The ability of Col VI to promote neural crest cell attachment and migration was tested in vitro using quantitative assays for these processes. Both native microfilaments and isolated tetramers of Col VI strongly promoted neural crest cell attachment and migration. Optimal stimulation of neural crest cell adhesion and migration was dependent upon structural integrity of Col VI since unfolded and disassembled alpha chains only weakly promoted cell attachment and were virtually inactive in supporting cell movement. The importance of a native macromolecular organization of Col VI further was analyzed in experiments in which dissociated tetramers were reassociated by Ca(2+)- and temperature-dependent self-aggregation. In contrast to native microfilaments, these oligomeric complexes were less effective in promoting neural crest cell movement, but still retained the ability to stimulate maximal cell attachment. The results indicate that Col VI is a primary component of the extracellular matrix deposited along neural crest migratory pathways, where it may participate in the regulation of cell movement by functioning as a migratory substrate. The ability of Col VI to promote neural crest cell adhesion and motility is highly dependent upon maintainance of a native macromolecular arrangement.

Emilin, a component of elastic fibers preferentially located at the elastin-microfibrils interface

The fine distribution of the extracellular matrix glycoprotein emilin (previously known as glycoprotein gp115) (Bressan, G. M., I. Castellani, A. Colombatti, and D. Volpin. 1983. J. Biol. Chem. 258: 13262-13267) has been studied at the ultrastructural level with specific antibodies. In newborn chick aorta the protein was exclusively found within elastic fibers. In both post- and pre-embedding immunolabeling emilin was mainly associated with regions where elastin and microfibrils are in close contact, such as the periphery of the fibers. This localization of emilin in aorta has been confirmed by quantitative evaluation of the distribution of gold particles within elastic fibers. In other tissues, besides being associated with typical elastic fibers, staining for emilin was found in structures lacking amorphous elastin, but where the presence of tropoelastin has been demonstrated by immunoelectron microscopy. This was particularly evident in the oxitalan fibers of the corneal stroma, in the Descemet's membrane, and in the ciliary zonule. Analysis of embryonic aorta revealed the presence of emilin at early stages of elastogenesis, before the appearance of amorphous elastin. Immunofluorescence studies have shown that emilin produced by chick embryo aorta cells in culture is strictly associated with elastin and that the process of elastin deposition is severely altered by the presence of antiemilin antibodies in the culture medium. The name of the protein was derived from its localization at sites where elastin and microfibrils are in proximity (emilin, elastin microfibril interface located protein).

Stable expression of chicken type-VI collagen alpha 1, alpha 2 and alpha 3 cDNAs in murine NIH/3T3 cells

As a component of an extensive network of microfibrils interwoven with large collagen fibers and in close contact with cell surfaces, type VI collagen plays an important role in cell-matrix interactions. To investigate the behaviour of chicken type VI collagen chains in heterologous host cells as a means to understanding the pattern of assembly of this collagen, we transfected murine NIH/3T3 cells with cDNAs encoding chicken alpha 1(VI), alpha 2(VI) and alpha 3(VI) chains. Cell lines that constitutively expressed the individual chains were analyzed by metabolic labeling and immunoprecipitation with specific antibodies. No self-association was observed for either alpha 1(VI) or alpha 2(VI) chains which were secreted as monomeric polypeptides. Furthermore, neither the chicken alpha 1(VI) nor alpha 2(VI) chains associated with the endogenous murine chains to form chimeric chicken/murine heterotrimers. In contrast, chimeric chicken/murine heterotrimers were detected in cell lines transfected with chicken alpha 3(VI) cDNA. These chimeric forms appeared to be properly aligned since their triple helices were stable to pepsin digestion. In addition, the chimeric heterotrimers coassembled and gave rise to disulfide-linked type VI collagen molecules.

Multiple forms of chicken alpha 3(VI) collagen chain generated by alternative splicing in type A repeated domains

Type VI collagen is a structurally unique component widely distributed in connective tissues. Its molecular structure consists of monomers that have the potential to assemble intracellularly into dimers and tetramers which, once secreted, can form microfilaments by end-to-end association. Individual monomers are composed of chains of Mr = approximately 140,000 (alpha 1 and alpha 2) and greater than 300,000 (alpha 3). Type VI collagen molecules contain a short triple helix with large globular domains at both ends. These domains are made for their greatest part of repetitive units similar to type A repeats of von Willebrand Factor. The alpha 3(VI) chain, contributing most of the mass of the NH2-terminal globule, appeared heterogenous both at the mRNA and protein level. Several alpha 3(VI)-specific clones that lack the sequences corresponding to repeats A8 and A6 were isolated from a chicken aorta cDNA library. Northern blot hybridization of poly (A+)-enriched RNA from chicken gizzard with cDNA fragments corresponding to several individual type A repeats showed that A8- and A6-specific probes did not hybridize to the lower Mr transcripts. Clones spanning approximately 20 kb of the 5'-end of the alpha 3(VI) gene were isolated from a chicken genomic library and subjected to analysis by restriction mapping, Southern blotting, and selective sequencing of the intron-exon boundaries. At the most 5'-end of the gene an additional type A repeat (A9), previously undetected in cDNA clones, was identified. Furthermore, it was determined that the presumed signal peptide and repeats A9 through A6 are encoded within individual exons. Reverse transcription and polymerase chain reaction of aorta RNA suggested that a mechanism of alternative mRNA splicing by a phenomenon of exon skipping generates alpha 3(VI) isoform variants that contain different numbers of type A repeats. Immunohistochemistry of frozen sections of chicken embryo tissues with repeat-specific mAbs showed that an antibody directed against a conditional exon has a more restricted tissue distribution compared to an antibody against a constitutive exon.

Structural and functional features of the alpha 3 chain indicate a bridging role for chicken collagen VI in connective tissues

Type VI collagen is a component of 100 nm long periodic filaments with a widespread distribution around collagen fibers and on the surface of cells. It is an unusual collagen constituted by three distinct chains, one of which (alpha 3) is much larger than the others and is encoded by a 9-kb mRNA. The amino acid sequence of the alpha 3(VI) deduced from the present cDNA clones specifies for a multidomain protein of at least 2648 residues made of a short collagenous sequence (336 residues), flanked at the N-terminus by nine 200 residue long repeating motifs and at the C-terminus by two similar motifs that share extensive identities with the collagen-binding type A repeats of von Willebrand factor. Type VI collagen and alpha 3(VI) fusion proteins bound to insolubilized type I collagen in a specific, time-dependent, and saturable manner. The alpha 3(VI) chain has three Arg-Gly-Asp sequences in the collagenous domain, and cell attachment was stimulated by the triple helix of type VI collagen and by alpha 3(VI) fusion proteins containing Arg-Gly-Asp sequences. This function was specifically inhibited by the Arg-Gly-Asp-Ser synthetic peptide. The type I collagen-binding and the cell-attachment properties of the alpha 3(VI) chain provide direct information for the role of type VI collagen in connective tissues.

Type VI collagen: high yields of a molecule with multiple forms of alpha 3 chain from avian and human tissues

A differential extraction procedure followed by molecular sieve column chromatography for the isolation of large quantities of the tissue form of type VI collagen is described. Recovery of the protein was more than 60% from both chick gizzard and human placenta. On reduced NaDodSO4-gels chick type VI collagen migrated as two major bands at Mr = 140,000 and 150,000 that were present in a 1:1 ratio and five less intense bands between Mr = 230,000 and 180,000. By immunoblotting with a polyclonal antibody against the pepsinized form of chick type VI collagen, all these bands were stained. Furthermore, the amino acid composition of the five higher Mr polypeptides indicated that they all contained hydroxyproline and hydroxylysine. In the chick type VI collagen molecule the five bands of higher Mr belong to the alpha 3 chain since they were recognized by monoclonal antibodies specific for the chick Mr = 260,000 alpha 3 chain. On examination of antigenic activity by solid-phase radioimmunobinding, densitometry of stained NaDodSO4 polyacrylamide gels, and protein content type VI was found to be an abundant collagen since it accounted for up to 0.1% of the tissue wet weight. The yields per tissue wet weight and the migration pattern of human type VI collagen polypeptides were similar to those of the chick. Agarose/polyacrylamide composite gels indicated that the molecular size of the tissue form of type VI collagen molecules under non-reduced conditions corresponded to a basic type of tetrameric molecule.